1. Field of the Invention
This invention relates to as-spun fibers made from sulfonated poly(p-phenylene terephthalamide) (PPD-T) with high inherent viscosity and to a process for making such fibers. The fibers of this invention exhibit unexpectedly high tenacity, both before and after heat treatment at elevated temperatures. The process of this invention yields PPD-T having a very high strength retention after heat aging and an unexpectedly high inherent viscosity in view of the conditions of sulfonation.
2. Description of the Prior Art
U.S. Pat. Nos. 4,075,269 and 4,162,346, issued Feb. 21, 1978 and Jul. 24, 1979 on the applications of R. S. Jones et al., disclose the spinning of fibers using sulfonated PPD-T of low inherent viscosity and low tenacity. The PPD-T is disclosed to be sulfonated using either concentrated sulfuric acid or fuming sulfuric acid and using specifically defined time and temperature conditions. The as-spun fibers of that invention exhibit single filament tenacities of only about 10-15 grams per denier and, after optimum heat treatment, of about 15-25 grams per denier.
U.S. Pat. No. 3,767,756, issued Oct. 23, 1973 on the application of H. Blades, discloses that fuming sulfuric acid can be used as a dope solvent with PPD-T; but contains caution that the temperature must be kept low and the time of exposure must be kept at a minimum because fuming sulfuric acid is known to degrade the polymer and result in a reduction in inherent viscosity and in fibers of reduced tenacity. Sulfonation of the PPD-T is not mentioned.
U.S. Pat. No. 3,671,542, issued Jun. 20, 1972 on the application of Kwolek, discloses that sulfuric acid having a concentration slightly higher than 100% can be used for a spinning solution solvent with PPD-T but the temperature of solution is about room temperature and the time for affecting solution is several days. Sulfonation of the PPD-T is not mentioned.
Japanese Patent Application Publication (Kokai) No. 50-16762 (1975) published Feb. 21, 1975 discloses dissolving PPD-T in 99.1% sulfuric acid for 17-20 hours at 75.degree.-80.degree. C. and for 14 hours at 90.degree.-100.degree. C. The polymer was degraded and fibers made from the polymer are disclosed to have a tenacity of about 15 grams per denier. Sulfonation of the PPD-T is not mentioned.